The present invention relates to a high-frequency circuit device, and more particularly to an improvement in a high-frequency circuit device in which a dielectric substrate made of ceramic is mounted on a grounding conductor made of metallic material.
For high-frequency signals ranging from VHF to microwave, it is common to employ a circuit using microstrip lines. An example of the structure of a substrate for such a circuit is illustrated in FIG. 1. A dielectric substrate 11 is provided at its bottom side with a metallized conductive film 12 along the bottom surface and a high-frequency circuit 13 of microstrip line formed along the top surface. As shown in FIGS. 1 and 2, film 12 on the bottom of the substrate 11 operates as a grounding conductor with respect to high frequency signals. A high-frequency circuit device is completed by mounting such a circuit substrate in a metallic casing 21 which has input and output connectors 22 and 23 at its both ends. In the mounting, the grounding conductive casing 21 must be sufficiently coupled to the conductive film 12 to allow high-frequency signals to flow freely between the casing and film. Such coupling is effected by joining the substrate 11 and the casing 21 by means of set screws, solder or electrically-conductive bonding agent. The selection of the joining means depends mainly on the material, and dimensions of the dielectric substrate, and reliability and cost of the high-frequency circuit device. In particular, when the dielectric substrate 11 is made of ceramic, solder or conductive bonding agent is preferably employed for the coupling.
The use of solder or bonding agent has an advantage that the substrate 11 can be mechanically and firmly integrated with the casing 21, but has a disadvantage that the substrate is likely to be cracked due to the great difference of linear thermal expansion coefficient between the ceramic material of the substrate 11 and the metallic material of the casing 21, thereby undesirably causing thermal stress in the substrate. More particularly, ceramic has a linear thermal expansion coefficient of 5 to 10 ppm/.degree. C., while metal used as the casing 21, such as copper, brass or aluminum is about 2 to 5 times as large as that of ceramic. Therefore, the expansion coefficient difference between the both materials is considerably large. Further, ceramic is mechanically fragile and expecially weak in pulling stress. For these reasons, if the substrate 11 of ceramic is joined directly to the casing 21 of metallic material with the use of solder or conductive bonding material, then the substrate tends to crack in the heating and cooling processes. In order to avoid this, there has been used an alloy which has almost the same linear thermal expansion coefficient as ceramic, for example, Kovar as the grounding conductor material (Kovar is a trademark of Westinghouse Electric Corporation for an iron-nickel-cobalt alloy. However, since this sort of special alloy is low in machinability and high in material cost, it is impossible to attain a less costly device even on a mass-production basis.